A typical photoflash lamp comprises a hermetically sealed glass envelope, a quantity of combustible material located in the envelope, such as shredded zirconium or hafnium foil, and a combustion supporting gas, such as oxygen, at a pressure well above one atmosphere. The lamp also includes an electrically or percussively activated primer for igniting the combustible material to flash the lamp. During lamp flashing, the glass envelope is subject to severe thermal shock due to hot globules of metal oxide impinging on the walls of the lamp. As a result cracks and crazes occur in the glass and, at higher internal pressures, containment becomes impossible. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to apply a protective lacquer coating on the lamp envelope by means of a dip process. To build up the desired coating thickness, the glass envelope is generally dipped a number of times into resin, typically cellulose acetate. After each dip, the lamp is dried to evaporate the solvent and leave the desired coating of cellulose acetate, or whatever other plastic resin is employed.
Another approach to providing a more economical and improved containing vessel is described in U.S. Pat. No. 3,893,797, wherein a thermoplastic coating, such as polycarbonate, is vacuum formed onto the exterior surface of the glass envelope. The method of applying the coating comprises: placing the glass envelope within a preformed sleeve of the thermoplastic material; drawing a vacuum in the space between the thermoplastic sleeve and the glass envelope; and simultaneously heating the assembly incrementally along its length, whereby the temperature and vacuum cause the thermoplastic to be incrementally formed onto the glass envelope with the interface substantially free of voids, inclusions and the like. Although this method provides an optically clear protective coating by means of a significantly faster and safer manufacturing process which may be easily integrated on automated producted machinery, it does present the disadvantage of requiring preformed plastic sleeves which must be individually designed for each different lamp type, made or purchased, stocked, and fed into the production apparatus which applies the sleeves onto the envelopes.
Further approaches toward providing improved protective coatings of lamps are described in the aforementioned Dow et al patent applications, which relate to coatings including UV curable photopolymers. For example, in the Dow et al application Ser. No. 753,255, a method of coating a flashlamp with a photopolymer is described comprising the following steps. First, the lamp is held vertically with the press up and dipped into a vat of the photopolymer at 60.degree. C. and extracted very slowly, the dip process taking about 45 seconds. The resulting coating thickness is about 0.020 inch. According to an alternative method described in the same Dow et al application, the flashlamp, while revolving, is sprayed with the liquid photopolymer and then transferred directly into the ultraviolet lamp chamber. Dow et al application Ser. No. 699,139 is somewhat similar except that either long or short strands of fiber glass are employed to reinforce the photopolymer coating.
An immersion process for applying a UV-cured coating on a photoflash lamp is also described in a published Japanese patent application identified as Public Disclosure Number 52-7720 and having a publication date of Jan. 21, 1977; the corresponding U.S. application Ser. No. 592,194, filed June 17, 1976 was abandoned, and a continuation of a continuation-in-part thereof issued as U.S. Pat. No. 4,076,489 on Feb 28, 1978.
A somewhat critical aspect of the aforementioned UV-cured coatings is that the shape and uniformity of thickness depends on the flow characteristics of the photopolymer resin as influenced by the force of gravity, orientation of the lamp after coating, and viscosity of the resin. Change in resin viscosity resulting from changes in temperature affect both the repeatability of the shape of the coating and the uniformity of thickness. These irregularities are retained once the coating is hardened. In the case of UV-cured coatings used to protect flashlamps from rupture at the time of flashing, thin coating regions resulting from improper resin distribution can result in containment failures. The comparative integrity or containment for various types of vessel constructions can be evaluated by the use of special test lamps, such as described in U.S. Pat. No. 3,955,912 assigned to the present assignee, which controllably induce bursting of the lamp upon ignition.
In another aspect, U.S. Pat. No. 4,197,333 issued to Leach et al suggests a flow-dispensing method for applying a containment coating to a flashlamp envelope. This flow-dispensing method includes the steps of: holding the lamp with its longitudinal axis disposed horizontally and rotating the lamp about its longitudinal axis; flow-dispensing a liquid photopolymer coating material having a viscosity during application in the range of 3000 to 5000 centipoise onto the envelope of the rotating lamp from dispensing means located above the lamp; and allowing the coating on the lamp envelope to be cure-hardened. Preferably, the rotating lamp is held in a fixed position below a fixed dispensing means having a plurality of needles through which the liquid coating material is dispensed. The needles are arranged along the length of the lamp and spaced a substantially fixed distance above a surface profile of the lamp envelope. The method is particularly useful for applying photopolymer coatings on lamps which are subsequently cure-hardened by irradiation with a source of ultraviolet light.
Rotation of the horizontally disposed lamp while coating material is dispensed from a plurality of selectively disposed needles located above the lamp exhibits surprising effectiveness in stabilizing the coating shape, once applied, for periods of up to thirty seconds prior to exposure for cure-hardening. This method is particularly effective for overcoming the viscosity characteristics encountered when using coating material of UV-curable photopolymers. Further, the complete coating can be applied in from one to four revolutions of the lamp, which is about 0.6 second or less. Uniform, repeatable coatings can be applied at production line speeds, and when applied to photoflash lamps, the resulting protectively coated vessel exhibits a superior containment capability, along with excellent photometric characteristics. In addition to the above-mentioned improvements in the resulting product, the method of applying lamp coatings according to the invention provides several advantages to the lamp manufacturing process. For example, the process can be solvent free; it requires a minimum of floor space; and it can be readily adapted to automated lamp production apparatus. Further, cure time is reduced to periods of less than a minute. A hard cure is effected immediately, without the need for warehousing to assure a complete cure.
However, it is to be noted that the envelope of the more common flashlamp configurations tend to exhibit portions which are more prone to failure than other portions. For example, the glass-to-metal sealing area of the flashlamp, usually referred to as the press portion, includes one or more electrical conductors which pass through and are sealed into the glass envelope. Thus, the glass is "worked" in this press portion causing undesired stresses and rendering the press portion more failure-prone than those areas which are not significantly altered or "worked".
Additionally, it is to be noted that areas of increased stress in the flashlamp are encountered due to the structural configuration. Specifically, the positional location of the ignition means and combustible materials within the envelope of the flashlamp tend to exert increased stresses and strains on the adjacent press portion. Thus, the "worked" press portion plus additional forces such as impinging glass particles tend to build undesired stress into the press portion of the flashlamp. Moreover, such stress areas in the press portion tend to be more failure-prone than other portions of the lamp which necessitates added containment capabilities for these failure-prone areas.